Structure and function of the peripheral and autonomic nervous systems
The peripheral and autonomic nervous systems are formed by nerve cell bodies and processes, i.e. axons and dendrites forming bundles, innervating the skin, skeletal muscles, glands and related structures. The nerve cell bodies are situated in the brain, spinal cord or in ganglia. Each myelinated nerve fiber is enveloped by Schwann cells while in cases of unmyelinated fibers several axons are enclosed in each Schwann cell. The Schwann cells are enveloped by basement membrane, some extracellular matrix and an endoneurial mesenchymal sheath. Many such units form a nerve, which is limited by perineurium of collagen, fibroblasts and related cells. An epineurium encloses the whole nerve, mostly comprising several nerve fascicles. A blood-nerve barrier prevents plasma proteins and many other substances from unrestricted penetration among the nerve fascicles. Motor and sensory nerves have the same structure but differ with regard to the axon and myelin dimensions. This means that in a mixed peripheral nerve it is not possible by the morphological characteristics to state whether a single axon is afferent or efferent.
Autonomic nerve fibers, sympathetic and parasympathetic, are accompanying the sensory and motor nerve fibers as well as blood vessels.